This application claims the priority of German Application No. 100 23 954.4, filed May 16, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method of qualitatively ascertaining the position and degree of severity of chatter marks formed in a finely-machined surface of a workpiece.
Chatter marks may occur under certain circumstances during the machining of workpiece surfaces. They are disruptive, in particular when they occur during the final machining, that is the fine machining, of the workpiece surface. This is because they then remain on the workpiece and can have a disruptive effect during the operation of a machine containing this workpiece, in particular due to noise generation and/or impairment of the service life. The causes of chatter marks being produced have not been definitively clarified, in particular not for all fine-machining processes. The fine-machining process most frequently used is grinding. Here, chatter marks are particularly disruptive when they extend axially over the entire width of the ground workpiece surface.
The problem with chatter marks has arisen in recent times in particular in the case of internal combustion engines or their camshafts, which interact with roller tappets. On the one hand, the cam surfaces sometimes have concave portions, that is, turning points in the cam profile, at which the risk of chatter marks forming is particularly acute. On the other hand, roller tappets react with particular sensitivity to chatter marks by producing noise. Flat-base tappets are much more tolerant in this respect.
Although the problem of chatter marks appears to be most urgent where camshafts are concerned, chatter marks likewise occur on other finely-machined machine parts, where they are likewise disruptive. For example, chatter marks on bearing surfaces lead to bearing vibrations, not only in the case of rolling bearings, where this is obviously so, but also in the case of sliding bearings. Bearing vibrations of this type manifest themselves in a corresponding development of noise. Any chatter marks would also be disruptive on synchronization rings of manual transmissions. They have a disruptive effect there if they impair the synchronization effect and consequently allow the shifting dogs to come into contact when they undergo relative rotation, which is not only noisy but also detrimental to their service life.
Chatter marks are instances of waviness directed transversely with respect to the machining direction in the surface of a workpiece. The elevation of the waviness of the chatter marks, that is their amplitude, is disruptive even if they occur to a relatively slight extent, i.e. if the amplitude of the chatter marks is much less than the permissible random surface roughness. The periodicity of disruptive chatter marks is consequently superimposed very much by random roughness structures, which makes it more difficult for chatter marks to be sensed by measuring instruments. Added to this is the fact that the chatter marks are—as stated—directed transversely to the instances of machining roughness, for which reason they cannot be ascertained by the tracing stylus method because of the lateral sensitivity of tracing styluses.
German Patent document DE 197 40 141 C1 describes a method of ascertaining a twisting structure in the surface roughness of a finely-machined shaft journal, using a tracing stylus method. As a difference in comparison with the chatter marks mentioned at the beginning, the instances of waviness of a twisting structure are directed in the circumferential direction, that is, approximately parallel to the instances of machining roughness, and can therefore be mechanically felt in the axial direction. Furthermore, comparatively only a small information density is required for ascertaining the superficial twisting structure, so that the relatively slowly operating mechanical feeling method can still be used with a reasonable time expenditure.
There are various known types of high-resolution surface-measuring methods with which a data record representing the actual microstructure or microtopography of a workpiece surface can be generated. Firstly, the interferometry method, in particular that of white-light interferometry, comes into consideration in this connection as a method of representing the microstructure of a finely-machined surface. Secondly, data on a surface microstructure can also be ascertained by the method of microstrip projection. Devices based on these measuring methods are commercially available.
On the basis of the prior art described above, the object of the invention is to present a method of ascertaining chatter marks in a finely-machined surface of a workpiece extending in the circumferential direction, with which at least the extent of the degree of severity of any chatter marks, but preferably also their circumferential position on the circumference of the workpiece, can be qualitatively and objectively determined. A qualitative indication in this respect is important on the one hand for quality assurance purposes, i.e. it must be possible to determine with respect to the presence of any chatter marks a characteristic assessment number which represents the extent of any chatter marks present on a particular workpiece surface. On the other hand, it is significant with regard to the research into causes of the occurrence of chatter marks in the case of particular final machining processes and with regard to production monitoring that chatter marks occurring can be localized and can be objectively and reproducibly quantified with respect to their degree of severity.
This object is achieved according to the invention by providing a method of qualitatively ascertaining the degree of severity and/or circumferential position of any chatter marks in a fine-machined surface of a workpiece extending in the circumferential direction. Firstly, a data record—referred to hereafter as the “ideal-form data record”—is generated from the idealized macroform of the workpiece surface in a data-processable form with the surface data of a type and density corresponding to the type and density of the surface data generated in the subsequent method step. Then, by means of a contactlessly operating measuring method which senses the microtopography of the workpiece surface two-dimensionally in the form of a close grid of surface points and resolves this microtopography at least to the depth of the actual surface deviations such as instances of waviness and/or roughness, a data record representing this microtopography is generated from the actual workpiece surface, at least in the region in which chatter marks are presumed,—hereafter referred to as the “actual-form data record”. The position of the surface points contained in the recorded grid is described by their respective spatial coordinates. A new data record—hereafter referred to as the “flat-form raw-data record”, which represents a seemingly unrolled, stretched-out-flat form of the microtopography of the workpiece surface, is generated by subtraction of the coordinate values directed orthogonally with respect to the workpiece surface—referred to hereafter as “Z coordinates” for short—of mutually corresponding surface points from the data of the ideal-form data record from the Z coordinates of the actual-form data record. The flat-form raw-data record is subjected to a digital in-phase bandpass filtering with respect to the instances of waviness directed in the circumferential direction of the workpiece surface of the data sequence of the Z coordinates, those instances of waviness of which the period length lies in the range of the period length of the chatter marks to be expected preferably being allowed to pass. Consequently, a further data record—hereafter referred to as the “flat-form filter-data record”—is generated, representing a seemingly unrolled, stretched-out-flat form of the microtopography of the workpiece surface that has been corrected to eliminate random instances of roughness. The flat-form filter-data record is for its part subjected to a stationary multiple cross correlation, in that the Z coordinates of the surface points lying in the same position in the circumferential direction of the workpiece surface are in each case multiply correlated with one another, generating a further, but now only two-dimensional, data record—referred to hereafter as the “chatter-mark data record”—, which represents the circumferential position of any chatter marks on the workpiece surface and their degree of severity.
The main idea of the invention is the special type of evaluation of the surface data, leading to the qualitative indication concerning the degree of severity and/or circumferential position of any chatter marks.
Accordingly, a contactlessly operating, high-resolution surface-measuring method is used to generate from the workpiece surface a data record representing the actual microstructure of the workpiece surface and to take away from this a data record representing the idealized macroform of the workpiece surface and in this way to generate a data record representing the flat-extended form of the microstructure of the workpiece surface. This data record is subjected to a digital in-phase bandpass filtering with respect to its circumferential waviness, the instances of waviness of interest here of the chatter marks preferably being allowed through. The data record formed in this way is for its part subjected to a stationary multiple cross correlation with respect to the transverse direction. This produces an only two-dimensional data record representing the circumferential position of any chatter marks on the workpiece surface and their degree of severity.
Expedient refinements of the invention are described herein.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.